The invention relates to a method of metering very small quantities of a non-flowable or poorly flowable powder, and to a corresponding apparatus therefor.
Many fine-grained powders are distinguished inter alia by the fact that they are either poorly flowable or completely non-flowable, but in any case they flow irregularly. Owing to the poor or very irregular flow of the powder, considerable difficulties arise in the accurate metering especially of very small quantities of such a powder. When the powder is introduced into a metering container or into a metering chamber, it often flows into the metering chamber only in surges, because the grains of powder adhere either to one another or to the surface of a feed device. Filling funnels for introducing the powder into the metering chamber therefore easily become blocked. This means that when the metering chamber is emptied at regular intervals, the quantity of powder located therein may vary very considerably. As long as the amount of powder supplied is very small owing to adhesion, the amount of powder passing into the metering chamber during the time in which the metering chamber is to be filled will also be only small. On the other hand, when there is a surge in the powder fed to the meeting container, i.e. when the blockage in a blocked filling funnel frees itself, the metering container will overflow, so that a suitable overflow container has to be provided. In the case of metering methods in which it is especially important that the quantity of powder metered should always be constant, or in which only very slight fluctuations in the relative weights of the metered quantities of powder are permissible, for example in the metering of pharmaceutical powders, the poor flow properties of the powder are especially disadvantageous since, in view of the subsequent administration of the metered quantity of powder, it is essential that the same quantity of powder should always be discharged from the metering chamber. This means that level indicators may be required for the metering chamber in order to ensure that the same quantity of powder is always discharged from the metering chamber, and this involves not inconsiderable expenditure.
One possible method by which poorly flowable or non-flowable powder can be rendered flowable, and hence metering can be simplified, consists in adding a so-called "lubricant" to the powder. Owing to adhesion forces between the surface of the fine powder grains and the surface of the lubricant, such lubricants form flowable powder/lubricant conglomerations. However, this method is disadvantageous in two respects. Firstly, for example in the case of so-prepared therapeutic agents for the treatment of human lungs, in many cases the lubricants cannot be dispersed by the "one-way system" lung; secondly, the conglomerations so formed are too coarse to permit sufficiently accurate metering of very small quantities. An example of such a lubricant that cannot be dispersed by the lungs is stearic acid (solid phase). When liquid lubricants are used to form the conglomerations, there do exist drying processes which can be used to remove the lubricants from the conglomerations again after conglomeration formation, but, depending on the drying process used, either the dried conglomerations then immediately disintegrate again into non-flowable fine powder, or the conglomerations are very hard; in the former case, the drying process results in the same problem of transporting poorly flowable fine powder grains, and in the latter case it is not possible to comminute the conglomerations although, in view of the intended method of administration (e.g. powder inhalation), comminution is essential because the conglomerations are too coarse in the uncomminuted state. Furthermore, it has also been found that when liquid lubricants are added, the size of the crystals of the individual fine powder grains changes so that, even if it were possible to comminute the conglomerations after drying, the powder grains would still be too coarse for inhalation.